1. Field of the Invention
This invention relates to helicopter blades and more particularly to torsionally flexible helicopter blades selectively shaped to effect blade torsional stability.
2. Description of the Prior Art
In the past, helicopter blades have been made relatively inflexible so that minimum torsional excursions of the blade in operation were encountered. U.S. Pat. No. 2,754,918 is typical of such prior art construction. With advances in technology and the attendant demand for improved performance, helicopter blades are now designed with thin airfoils generally diminishing in thickness between the blade root and the blade tip for improved aerodynamic performance. Further, many helicopter blades are currently made of composites with their inherent low shear stiffness properties. In view of the thin tip airfoil and the composite construction, the modern helicopter blade is torsionally compliant, meaning torsionally flexible, and the torsional deflections or excursions at the blade tip are primarily due to the elastic twisting of the rotor blade structure due to the imposition of aerodynamic or inertia loads thereon. A typical example of such a torsionally compliant modern helicopter blade is shown in Fenaughty U.S. Pat. No. 3,874,820 and U.S. Pat. Nos. 3,728,045 and 3,822,105.
Efforts have been made to introduce torsional stability into such torsionally compliant blades, but none have been fully satisfactory. For example, blades have been made with a swept tip as shown in U.S. Pat. No. 3,721,507 in an effort to produce torsional stability but such swept tip constructions have the inherent disadvantage that they generate lift in the swept portion of the blade at a station displaced from the blade feathering axis and thereby impose loads on the blade control system during all modes of operation.
The selective positioning of the elastic axis is known in the fixed wing sailplane art wherein in U.S. Pat. No. 3,561,702 the fixed sailplane wing is swept aft of the elastic axis of the wing root to produce twisting of the wind about this axis due to changes in lift. This sailplane patent twists the total wing about the wing root, with the region of maximum twisting located adjacent the root. Contrary to this sailplane teaching, my invention involves tailoring the structural shear centers locally along the blade span to provide local twisting of these sections due to airloads generated outboard of the sections. In particular the shear centers are tailored to be forward of the center of lift at the blade tip so that the torsionally flexible tip sections twist nose down locally due to changes in upward lift, or vice versa. With my construction the changes in lift produce no appreciable twisting moment about the blade root sections since at these sections the centers of lift and the centers of shear are coincident and hence no control loads are created by my invention. The swept wing sailplane of U.S. Pat. No. 3,561,702 can be likened to the swept wing helicopter blade discussed hereinafter.
The purpose of this invention is to effect torsional stability in a torsionally compliant helicopter rotor blade.